1. Field of the Invention
The present invention relates to an apparatus for applying a coating liquid to a surface of a thin plate by means of a coating roll and particularly to a roll coating apparatus for applying a coating liquid having a high viscosity such as photoresist or polyimide resin with a uniform thickness to a surface of a thin substrate such as a printed circuit board, a glass substrate for a liquid crystal display panel or a substrate for an image sensor.
2. Description of the Background Art
An apparatus for forming a layer of a coating liquid on a surface of a thin substrate such as a printed circuit board by using a coating roll is disclosed for example in Japanese Utility Model Publication No. 42133/1984 entitled "Coating Apparatus for a Liquid of a High Viscosity", or Japanese Patent Laying-Open No. 230658/1985 entitled "Coating Apparatus for Applying a Photosensitive Liquid to a Printed Circuit Board".
FIG. 1 is a perspective view showing schematically a typical example of such a conventional apparatus. FIG. 2 is a sectional view in the direction of the arrow II in FIG. 1. Referring to FIGS. 1 and 2, the conventional apparatus comprises a coating roll 30 pressed against the upper surface of a substrate p transported in the direction of the arrow D, a doctor roll 32 extending along an axis of the coating roll 30 and contacting the coating roll 30 on a side thereof, a backup roll 34 extending under the coating roll 30 along its axis and in contact with a lower side of the coating roll 30, for supporting the substrate p inserted between the rolls 30 and 34, and a coating liquid supply nozzle 36 provided movably along the axis of the coating roll 30 over a portion of contact between the coating roll 30 and the doctor roll 32, for supplying a coating liquid 42 to the coating roll 30. The coating roll 30 and the backup roll 34 rotate according to movement of the substrate p. The doctor roll 32 is rotatable around its axis. The portion of contact between the doctor roll 32 and the backup roll 34 constitutes a liquid reservoir 38.
FIG. 3 is a sectional view taken along the line III--III in FIG. 2, showing a section of a peripheral surface of the coating roll 30. As shown in FIG. 3, the circumferential surface of the coating roll 30 is provided with a plurality of grooves 40 along the circular direction of the coating roll. According to the disclosure of Japanese Patent Laying-Open No. 230658/1985, the width W of each groove 40 is in the range from 0.1 mm to 3 mm and preferably W=0.5 mm. The distance F between adjacent grooves 40 is in the range from 0.1 mm to 5 mm and preferably F=0.5 mm.
Referring to FIGS. 1 to 3, operation of the conventional apparatus will be described. The coating liquid supply nozzle 36 reciprocates along the axial direction of the coating roll 30, to supply the coating liquid 42 to the liquid reservoir 38. The coating roll 30 rotates with a prescribed number of revolutions per minute, so that the coating liquid 40 adhering to a peripheral surface of the coating roll 30 from the liquid reservoir 38 is uniformly applied to the surface of the coating roll 30.
The substrate p moves in the feeding direction D and it is inserted between the coating roll 30 and the backup roll 34. The coating liquid spreading around the surface of the coating roll 30 adheres to the substrate p as a result of contact between the coating roll 30 and the surface of the substrate p, whereby a film 44 is formed. The coating liquid supply nozzle 36 supplies the coating liquid 42 continuously to the liquid reservoir 38 while reciprocating along the axial direction of the coating roll 30, over the liquid reservoir 38. Accordingly, the film 44 is successively formed on surfaces of a plurality of substrates p transported.
FIG. 4 is a perspective view showing schematically another example of a conventional apparatus. Referring to FIG. 4, this apparatus comprises a doctor bar 46 which extends along the axial direction of the coating roll 30 and contacts a side surface of the coating roll 30, in place of the doctor roll 32 of the apparatus shown in FIGS. 1 and 2. The doctor bar 46 has a cut in a portion contacting the coating roll 30, whereby a liquid reservoir 48 is formed.
The apparatus shown in FIG. 4 is also capable of forming a film 44 on the substrate p in the same manner as in the case of the apparatus shown in FIGS. 1 and 2.
The conventional apparatus shown in FIGS. 1 to 4 have, however, disadvantages as described below.
One of the disadvantages is that when a coating liquid having a high viscosity, e.g., 1,000 cp or more at a normal temperature, is applied to a substrate, the surface of the film formed on the substrate is not smooth in some conventional apparatus. For example, the coating liquid used in the coating apparatus described in the above mentioned Japanese Patent Laying-Open No. 230658/1985 is an etching-resisting photosensitive liquid (photoresist for etching) and it has a viscosity of about several tens to hundreds of cp. On the other hand, a solder-resisting photosensitive liquid has a viscosity of about 20,000 cp or more at a liquid temperature of 20.degree. C. If a solder-resisting photosensitive liquid is applied to a surface of a substrate by using a conventional roll coating apparatus, it is known that the surface of the film obtained is not uniform due to the high viscosity. Moreover, it is also known that there are some parts of the surface of the substrate, where the film is not formed.
Polyimide resin used as an interlayer insulating film for a liquid crystal glass substrate or other substrate has a viscosity of several thousands of cp at a liquid temperature of 28.degree. C. If polyimide resin is applied to the surface of the substrate using the coating roll 30 having grooves as shown in FIG. 3, a film having a non-uniform surface or a partially deficient film is obtained.
FIG. 5 is an enlarged view of a part of the conventional apparatus shown in FIG. 2. Referring to FIG. 5, the reason for the formation of the above mentioned non-uniform or incomplete film will be described. If the viscosity of the coating liquid 42 is increased, the coating liquid 42 adheres not only to the coating roll 30 but also to the doctor roll 32 under the position of contact between the coating roll 30 and the doctor roll 32. A required quantity of the coating liquid 42 does not adhere to the surface of the coating roll 30. As a result, it is difficult to form the film 44 having a required thickness on the surface of the substrate p.
Threadlike portions 54 of the coating material are generated between the coating liquid 50 adhering to the coating roll 30 and the coating liquid 52 adhering to the doctor roll 32. Due to those threadlike portions 54, unevenness is caused in the thickness of the coating film 44 formed on the surface of the substrate p.
Threadlike portions 56 of the coating material are generated also between the surface of the coating roll 30 and that of the substrate p after the coating roll 30 has contacted the substrate p. As a result, the surface of the coating film 44 formed on the surface of the substrate p is not uniform.
As is evident from the above description, a film 44 having a uniform surface cannot be formed with a coating liquid having a high viscosity, e.g., a viscosity of 1,000 cp or more at a normal temperature on the substrate p by any of the conventional apparatus.
Another disadvantage is that small air bubbles are contained in the film 44 formed by the conventional apparatus. This phenomenon is caused by the threadlike portions 56 of the coating material generated between the coating roll 30 and the film 44. The threadlike portions 56 of the coating material become gradually thin according to the rotation of the coating roll 30 and the movement of the substrate p. When the threadlike portions 56 are broken finally, they are absorbed in the film 44 on the surface of the substrate p with air being taken therein. As a result, small air bubbles are formed in the film 44 on the surface of the substrate p.
A further disadvantage is that the thickness of the film formed on the substrate is sometimes not uniform according to the conventional techniques. The reason for this phenomenon will be described.
As shown in FIGS. 1 and 3, if a coating liquid of a high viscosity is applied to the substrate p, the coating liquid on the surface of the coating roll 30 is thin in a portion transferred to the substrate p and thick in the other portions. In order to compensate for this difference of thicknesses of the coating liquid on the surface of the coating roll 30, the conventional apparatus includes the coating liquid supply nozzle 36 shown in FIGS. 1 and 4.
The coating liquid supply nozzle 36 reciprocates along the axial direction of the coating roll 30. In consequence, the coating liquid is uniformly supplied to the whole of the liquid reservoir 38 or 48, so that the coating liquid can be applied uniformly to the surface of the substrate p.
However, after a long period of coating work, a large difference is caused in the viscosity of the coating liquid between the central portion 60 or 64 corresponding to the position of passage of the substrate p and both ends 62 or 66 of the reservoir.
The coating liquid supplied to the central portion 60 or 64 of the reservoir is applied continuously to the surfaces of the successively transported substrates p through the circumferential surface of the coating roll 30. The coating liquid in the central portion 60 or 64 of the liquid reservoir is constantly renewed. The viscosity of the coating liquid in this portion is maintained nearly constant as far as the viscosity of the coating liquid supplied from the coating liquid supply nozzle 36 does not change.
The coating liquid supplied to both end portions 62 or 66 of the liquid reservoir is not applied to the substrate p. Furthermore, if the viscosity of the coating liquid is high, the coating liquid does not move so much. Accordingly, the coating liquid in those end portions is renewed only locally. Particularly in the end portions 62 or 66 of the liquid reservoir, the coating liquid 48 enters the grooves 40 cut in the circumferential surface of the coating roll 30 as shown in FIG. 6. The coating liquid 40 in each groove 40 is hardly renewed. The solvent in the coating liquid 48 evaporates as the time proceeds. As a result, the viscosity of the coating liquid 48 in the end portions 62 or 66 of the liquid reservoir is increased.
A large difference in the viscosity of the coating liquid between the central portion 60 or 66 and the end portions 62 or 66 of the liquid reservoir does not exert significant effects if the width of the substrate p to be processed is constant. However, if the type of the substrate p is changed and the width thereof becomes large, the viscosity of the coating liquid which constitutes a film on the surface of the substrate p changes dependent on the position of the substrate p in the direction perpendicular to the feeding direction of the substrate p. As a result, a film of a uniform thickness cannot be formed.
In order to promote renewal of the coating liquid 68 in the end portions 62 or 66 of the liquid reservoir, it may be considered to use a method of increasing the quantity of the coating liquid supplied from the coating liquid supply nozzle 36 to a quantity more than necessary. Such a method involves, however, disadvantages such as deterioration of the coating liquid or increase of the size of the apparatus and therefore it is not a practical method.
On the other hand, in order to lower the high viscosity of the coating liquid, a method of diluting the coating liquid may be considered.
FIG. 7 shows a relation between a dilution ratio and a viscosity of liquid solder resist at about 25.degree. C. Referring to FIG. 7, the viscosity of this liquid at about 25.degree. C. is lowered from 125 ps to about 100 ps as a result of 1% dilution by using thinner.
However, the viscosity is not determined only by the dilution ratio. An important factor for the viscosity is temperature of the liquid solder resist. As is understood from FIG. 8, the viscosity changes considerably dependent on the temperature of the liquid solder resist. On the other hand, in order to form a film of a desired thickness on a substrate, the viscosity of the liquid solder resist needs to be maintained constantly at a predetermined value. In order to maintained the viscosity constant, it is necessary to control the dilution ratio appropriately for each case in view of the temperature of the liquid solder resist. However, this control is very troublesome.